Films of superconducting material, with thicknesses of from about 0.1 to about 500 microns, have been proposed for use in electronic circuits and superconducting devices. See, for example, an article by Peter E. Norris et al. entitled "In-situ thin films by MOCVD," Superconducting Industry, Vol. 3, No. 1, Spring, 1990.
The high Tc superconductors currently available are ceramic materials. It is known that ceramic materials may be fabricated into film by evaporation or by sputtering methods; however, both of these methods are usually conducted under vacuum. See, e.g., B. Oh et al., "Critical current densities and transport in superconducting YBaCuO films made by electron beam coevaporation," Applied Physics Letters 51, 852 (1987); M. Hong et al., "Superconducting Y-Ba-Cu-0 oxide films by sputtering," Applied Physics Letters 51, 694 (1987); D. Dijkkamp et al., "Preparation of Y-Ba-Cu oxide superconductor thin films using pulsed laser evaporation from high Tc bulk material," Applied Physics Letters 51, 619 (1987); and S. Witanachchi et al., "Deposition of superconducting Y-Ba-Cu-0 films at 400 degrees centigrade without postannealing," Applied Physics Letters 53, 234 (1988). The disclosure of each of these references is hereby incorporated by reference into this specification.
One of the more interesting high Tc superconductive materials has the formula (AO).sub.m M.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.2n+2, wherein A is selected from the group consisting of thallium, and mixtures of thallium and lead, m is from about 1 to about 2 (and generally is 1 or 2), M is selected from the group consisting of barium, strontium, and mixtures thereof, and n is at least 1. In one preferred embodiment, A is thallium, m is 2, M is barium, and n is 3; this composition has a critical temperature of about 122 degrees Kelvin. This material is well known to those skilled in the art and is disclosed, e.g., an article by A. W. Sleight entitled "Chemistry of High-Temperature Superconductors," Science, Volume 242 (Dec. 16, 1988, at pages 1519-1527), the disclosure of which is hereby incorporated by reference into this specification.
Because thallium is a toxic material, thalliumn-containing superconductors are often made by a two-step process in which a precursor which does not contain thallium is first prepared, and thereafter it is reacted with thallium. Thus, for example, a process for preparing a solid precursor of barium-calcium-copper oxide material and thereafter reacting it with thallium oxide is disclosed in an article by Z. Z. Sheng et al., "Tl.sub.2 O.sub.3 vapor process of making Tl-Ba-Ca-Cu-O superconductors," Applied Physics Letters, 53(26), 26 December 1988, the disclosure of which is hereby incorporated by reference into this specification.
Processes for making precursors of thin films containing the thallium-calcium-barium-copper oxide material are known to those in the art. See, for example, articles by D. G. Naugle et al., "Preparation of oriented high-Tc Tl-Ca-Ba-Cu-O thin films from co-condensed amorphous Ca-Ba-Cu alloys by post deposition oxidation and Tl diffusion, Journal of Applied Physics, 68(3), Aug. 1, 1989; and S. I. Shah et al., "Fabrication of Tl-Ba-Ca-Cu-O films by annealing rf-sputtered Ba-Ca-Cu-O films in thallium oxide vapors," Applied Physics Letters, 56(8), 19 February 1990. The disclosures of these references is incorporated by reference into this specification. The processes described in the aforementioned Naugle and Shah papers for making films consisting of superconductive material all require the use of a reduced pressure environment during the vapor deposition step. Such processes are not suitable for large scale production of the superconductive films. A substantial amount of energy, time, and money is required for a vacuum system and its operation; and such a system is not always suitable for large scale production. In addition, the size of the superconductive film which can be made by the vacuum deposition processes is limited by the size of the vacuum chamber used.
In copending patent application 07/528,147, applicants have described a one-step plasma deposition process in atmospheric environment for preparing yttrium-barium-copper, and bismuth-calcium-strontium-copper superconductive films. However, such a one-step atmospheric process is not suitable for the preparation of the thallium-containing superconductive materials because of the toxicity of thallium.
To the best of applicant's knowledge, no one has disclosed a plasma deposition process for making the precursor of the thallium superconductive material. Applicants believe that this might be due to the fact that the charge polarities of the ions comprising the precursor are not well understood by those skilled in the art.
It is known that, when a mixture of various compounds is subjected to the irradiation and heat of the radio-frequency plasma waves, a multiplicity of charged species or ions will be created at different rates; these species and/or ions may collide with each other and/or the plasma gas and/or the substrate, creating new particles. There is a substantial amount of uncertainty as to what species or ions or particles are generated with the plasma irradiation of a mixture of barium oxide, calcium oxide, and copper oxide. To the best of applicants' knowledge, there is no publication describing the ion spectra of emitted particles created by plasma irradiation of such mixture.
The rate of deposition of a charged species onto a substrate will vary with the charge polarity of the species and the electrical potential of the substrate. Because many mixtures create species with differing charge polarities upon irradiation with plasma waves, such mixtures do not necessarily deposit such species at a uniform rate upon any given substrate. Consequently, many films deposited by plasma deposition do not reflect the stoichiometry of the starting mixture of materials.
The problems associated with the non-stoichiometric deposition of reagents are illustrated in a paper by A. Shah et al., "rf plasma-generated superconducting Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x films," Applied Physics Letters 57(14), 1 October 1990, the disclosure of which is hereby incorporated by reference into this specification. In the process described by this paper, a mixture containing 1 part of yttrium, 2.05 parts of barium, and 2.76 of copper was provided. This mixture was then subjected to irradiation by plasma waves. However, the film so produced contained 1.0 part of yttrium, 2.0 parts of barium, and 3.0 parts of copper. Apparently a substantial amount of trial and error was necessary in order for the writers to be able to produce a composition with the desired 1:2:3 molar ratio.
It is an object of this invention to provide a process for the preparation of a precursor film containing calcium, copper, and either barium or strontium that will produce such film at a rate of at least 1 micron per minute on a substrate with a surface area of at least about 30 square centimeters.
It is another object of this invention to provide a process for the preparation of a precursor film containing calcium, copper, and either barium or strontium which does not require the use of a vacuum environment.
It is another object of this invention to provide a process for the preparation of a precursor film containing calcium, copper, and either barium or strontium that is able to readily produce such materials in large sizes and/or complicated shapes.
It is another object of this invention to provide a process for the preparation of a precursor film containing calcium, copper, and either barium or strontium that is substantially homogeneous.
It is another object of this invention to provide a process for the preparation of a precursor film containing calcium, copper, and either barium or strontium which will produce a film with substantially the same stoichiometry as the stoichiometry of the starting materials used.
It is another object of this invention to provide a process for the preparation of a precursor film containing calcium, copper, and either barium and/or strontium which is relatively economical and flexible.
It is another object of this invention to provide a process for the preparation of a precursor film containing calcium, copper, and either barium and/or strontium which contains a relatively large amount of small grains.
It is another object of this invention to provide a process for the preparation a thallium-containing superconductive material by diffusion of a thallium-containing vapor into a precursor film containing calcium, copper, and either barium and/or strontium.